Investigators combing through the scene of the deadly Boston Marathon terrorist attack have recovered a circuit board that they believe was used to detonate the bombs, according to an official briefed on the investigation.

Investigators have also recovered components of the bombs, enabling them to determine that they consisted of two 6-liter pressure cookers packed with nails, ball bearings, and other metal. The makeshift bombs were placed in black duffel bags, the official said.

The one that exploded first was placed on the ground on Boylston Street, across from the finish-line viewing stands where dignitaries, including Governor Deval Patrick, had been sitting earlier. The second bag was placed on the ground about 75 to 100 yards down the street, outside the Forum restaurant at 755 Boylston, according to the official.

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Just before 3 p.m. Monday, the bombs detonated about 12 seconds apart, killing three people and injuring 176 others, including 17 who are still in critical condition. The attacks created a scene of bloody carnage near the finish line of a race that is a colorful rite of spring in Boston and draws runners from around the world.

President Obama, speaking to reporters at the White House this morning, called the attacks a “heinous and cowardly act” and said that the FBI was investigating it “as an act of terrorism.”

“We will find whoever harmed our citizens and we will bring them to justice,” he said.

Governor Patrick announced late this afternoon that Obama would be coming to Boston Thursday to attend an 11 a.m. interfaith service at the Cathedral of the Holy Cross.

Eight-year-old Martin Richard of Boston was one of those killed in the attack. A second person who died in the blast was also identified today. She was Krystle Campbell, 29, of Arlington, her grandmother said. The third person killed in the blast was a Boston University graduate student, the university said this afternoon, though it did not release the student’s name.

Doctors at several major hospitals in Boston said they had seen evidence that the bombs contained some kind of shrapnel.

Krystle Campbell, 29, of Arlington was killed in the blast. (Facebook photo)

Dr. Ron Walls, an emergency physician from Brigham and Woman’s Hospital, said items were removed from several patients that “clearly were designed to be projectiles that were built into the device.” He described the items as small ball bearings and nails.

Similar comments were made by doctors at Massachusetts General Hospital, Boston Children’s Hospital, and Boston Medical Center.

A federal investigator who responded to the bombings also said authorities had recovered shrapnel that appeared to be ball bearings or BBs.

“Anything can cause shrapnel, even a soda can nearby, but it appears this was prepared shrapnel,’’ said the investigator, who insisted on anonymity because he wasn’t authorized to discuss the investigation.

“There appears to be beyond the normal stuff. It looks like the device was prepared with it.” He said agents were collecting evidence and taking it to the Hynes Convention Center.

Richard DesLauriers, special agent in charge of the Boston FBI office, said pieces of black nylon consistent with a black backpack or bag had been found, along with fragments of BBs and nails “possibly contained in a pressure cooker device.” He had no comment when asked whether a circuit board had been discovered.

He emphasized that the person who detonated the bombs is “someone’s friend, neighbor, coworker, or relative.”

He urged anyone to contact authorities who if they had heard someone talk threateningly about the marathon or the date of April 15, or knew someone with an unusual interest in explosive devices. He said someone might have heard the bombers testing the explosives in a remote area.

He also asked people who had attended the marathon to report if they had seen anyone carrying a large, heavy black bag that day. Authorities believe the bag would have been heavy due to the size of the explosion and the materials required.

“Someone knows who did this. Cooperation from the community will play a crucial role in this investigation,” DesLauriers said.

He said no one had claimed responsibility for the blasts. “The range of suspects and motives remain wide open,” he said.

Law enforcement officials today also asked the public to provide them with video and photographic evidence that could provide clues.

“I would encourage you to bring forward anything. You might not think it’s significant, but it might have some value to this investigation,” said Colonel Timothy Alben, superintendent of the Massachusetts State Police.

Officials emphasized that, contrary to rumors and reports on Monday, no other explosive devices had been found by authorities on Monday. DesLauriers said “there are no known additional threats.”

But the officials also said that a major investigation was underway, with local and state authorities getting the full backing of the federal government, including the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives.

They asked people for their cooperation as investigators fan out over the area, interviewing witnesses, and they asked for the public’s patience as security is stepped up and experts process the expansive crime scene in the Copley Square area, where the blasts occurred.

Martin Richard, 8 was also killed. (Reuters)

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“We want people to come and go. We want you to live your life,” said Boston Police Commissioner Edward F. Davis. But at the same time, he said, “We want you to be vigilant ... give us a little room in the Copley Square area.”

He said the Copley Square area was “the most complex crime scene in the history of the department.” The crime scene area was 15 blocks on Monday but had been reduced to 12 blocks today and “we will continue to collapse the crime scene,” he said. But he expected that the scene would be closed off “for another two days anyway.”

Davis said that there had been two sweeps of the marathon route by bomb technicians before the blast. More officers were also assigned to the race than ever before. But he also said the marathon was a “soft target” because there was open access to the race.

Law enforcement and elected officials, who held two major news conferences to brief the public, also thanked the police, firefighters, emergency medical workers, volunteers, and doctors and nurses who aided people.

DesLauriers commended the public for its assistance so far, saying authorities had received “voluminous tips.”

Overnight, a wave of law enforcement officials swarmed a home in Revere, looking at what the Revere Fire Department described as a “person of interest.’’ But by late this afternoon, sources told the Globe that the Saudi Arabian man who was being scrutinized was not a suspect.

“He’s just a victim. He appears not to be involved,” said one official briefed on the investigation.

President Obama, in his White House comments, lauded the stories of first responders and ordinary people racing to people’s aid with “heroism, and kindness, and generosity, and love.”

“The American people refuse to be terrorized,” he said. “If you want to know who we are, what America is, how we respond to evil, that’s it — selflessly, compassionately, unafraid.”

an Saltzman, John R. Ellement, Scott Allen, Brian Ballou, Andrew Ryan, Steven A. Rosenberg, and Akilah Johnson of the Globe staff, and Globe correspondent Evan Allen contributed to this report. John R. Ellement can be reached at ellement@globe.com. Follow him on Twitter @JREbosglobe.